Should a lot of political and political theory content be available on the substantive content on the game site?

Yes, in a politically-minded game this could be considered the game's "lore" and there will always be players that are interested in the lore of the game-world in which they're playing. It's important to make your message easily accessible for quick consumption, though. The average player will perhaps read one or two sentences and then move on, so in order to get players to understand the Manifesto, it must be presnted in bite-sized chunks that are quickly consumed and digested. This bite-sized information should be presented naturally through the flow of gameplay (or perhaps even presnted as gameplay mechanics itself). Longer messages and more in-depth political discussions can be made available for players who are (or become) truly interested in the game's meaning. These can be presnted on the web for more casual perusal.

It's interesting what you say about this being the game's "lore." I am reminded of the Civilopedia in Civilization, the in-game "lore book" that teaches not only history, but also technical aspects of units, states, etc. And I agree, certainly: just like any other game, lore should be dispensed as one plays the game, instead of being thrown willy-nilly. Dragon Age is also a good example of this: as one plays the game and encounters new things, those things get added into a journal for further, more-in-depth (very in-depth) reference. Players don't have to look at the reference, of course, only if they're interested. And if they're playing a game based on our comments on AgoraXChange, they probably will be interested in political theory. (And if they're not-- that's okay, too, but there will probably be severe cleavages between players who care about theory and players who don't. Kind of like real-life states!)